The purpose of this study was to develop a diagnostic system of jaw function which is able to record jaw motion with 6 degrees of freedom, multichannel EMG of jaw muscle including inferior lateral pterygoid muscle and temporomandibular sound simultaneously ; and to investigate the mechanism of TMJ sound with this diagnostic system. In the first years of this project, an intraorally recording method of inferior lateral pterygoid muscle activity with fine wire electrodes was established after a preliminary study in cadavers. Jaw motion with 6 degrees of freedom and EMG activity of inferior lateral pterygoid muscle were recorded simultaneously. This method had never before been attempted in the world. Eignt subjects were studied using this system with the following results :1. When the activity of inferior lateral pterygoid muscle was observed according to the forward translation of the condyle, the amount of its activity was influenced by the condyle's translation speed or position.2. An exception was observed during the slowest opening movement of mandible. The onset of activity of the lateral pterygoid muscle occurred at or before the beginning of forward translation of the ipsilateral condyle.Finally, TMJ sound recording method was established and the jaw function diagnostic system was completed. The diagnostic system is efficient in investigating the presence of condyle path path deflection, change of speed of condylar movement during clicking and the TMJ sound mechanisms as well.